Plants are subject to attack by a great number of pathogens. These pathogens can be, for example, bacteria, fungi, or nematodes. Pesticidal compounds have long been used to increase yields and extend agricultural production capabilities into new areas. They have also been extremely important tools for ameliorating season-to-season differences in yield and quality caused by weather-driven variations in disease pressure.
The future role of pesticides in agriculture is increasingly threatened by several factors including; the development of pest resistance, increasing concerns about food safety, and environmental accumulation of toxic compounds. As older pesticides are removed from the market due to regulatory changes, and new pesticides are becoming increasingly expensive to register, there is an increasing need to find ways to more wisely use the remaining, safest pesticides. This is particularly true for the many crop/disease combinations which do not represent large enough markets to pay for the cost of new compound registration. Wiser pesticide use will include ways to reduce application rates (and thus potential residues), finding ways to extend registrations to new crops, and identifying new compositions and treatments to combat the development of pest resistance.
Chemical pesticides have provided an effective method of control; however, the public has become concerned about the amount of residual chemicals which might be found in food, ground water and the environment. Stringent new restrictions on the use of chemicals and the elimination of some effective pesticides from the market place could limit economical and effective options for controlling pests. In addition, the regular use of chemical toxins to control unwanted organisms can select for resistant strains.
Alternative strategies to pesticide application are needed for the control of agriculturally important pests. Such strategies will help address public concern regarding pesticide pollution, as well as the perception that pesticide residues on food pose a threat to human health.
Among the more destructive zoosporic plant pathogens are the downy mildews (which are primarily associated with severe foliar diseases of many crops), and numerous species in the genus Pythium and Phytophthora which are destructive pathogens of roots, foliage, and fruits. Within the fungal genus Pythium are plant pathogenic species which can cause significant losses in vegetable production.
The value of vegetable crops grown in the state of Florida totaled over $1.79 billion for the 1991–92 crop season with much of the cabbage, cucumber, pepper, and tomato crops planted as transplants. Tomato alone accounts for $735 million in this production total and over 80% of the crop is transplanted. The use of vegetable transplants in commercial field production systems is important in many areas of the United States. In California, all of the celery, fresh market tomato and pepper, and most of the cauliflower and broccoli are grown as transplant crops. In Monterey County alone, the value of the transplants grown in 1992 amounted to nearly $18 million and represented a final crop value of $176 million.
Plant pathogenic Pythium species can kill a plant at the seedling stage or can reduce crop yield by destroying the root system of a mature plant. While diseases in the seedling stage are often controlled by fungicide application, the continued use of certain highly effective fungicides, e.g., metalaxyl, has faced regulatory uncertainty for use in vegetable transplant greenhouse production systems. In addition, continued use of a particular fungicide can result in the development of tolerance by the pathogen. Fumigants such as methyl bromide, which are routinely used on high cash-value crops, also face regulatory uncertainty. Thus, disease control (in particular damping-off) in the production greenhouses as well as in the field following transplanting are a major concern.
An alternative to the use of chemical pesticides for controlling phytopathogenic Pythium spp. is the use of biological control agents for vegetable transplants, a large and expanding industry in which disease protection is needed in the greenhouse as well as in the field after transplanting.
U.S. Pat. No. 4,574,083 to Baker and Lifshitz describes Pythium nunn, which is not pathogenic to plants and can protect seedlings from damping-off in greenhouse evaluations.
There are a number of studies examining the effect of seed treatment with oospores of P. oligandrum on reducing subsequent levels of disease, most of which have been conducted in the greenhouse. Deacon (1976; Trans. Br. Mycol. Soc. 66:383–391) described the ability of mycelial seed coatings on wheat to significantly reduce the disease incidence over untreated seeds.
Vesely (1977; Phytopath Z. 90:113–115; 1979) observed that application of oospores to sugarbeet seed reduced damping-off incidence to a similar level as thiram treatment (see also Schippers, B. and W. Gams, eds. Academic Press, Soil-Borne Plant Pathogens). In U.S. Pat. No. 4,259,317, Vesely et al. describe the application of Pythium oligandrum, or “Polygandron.”
Bacterial wilt caused by Ralstonia solanacearum (Rs) is a major disease problem in fresh tomato production fields in north Florida. Fusarium is also an important plant pathogen.
Damage to plants caused by nematodes is also a prevalent and serious economic problem. Nematodes cause wide-spread and serious damage in many plant species. Many genera of nematodes are known to cause such damage. Plant-parasitic nematodes include members of the Phylum Nematoda, Orders Tylenchida and Dorylaimide. In the Order Tylenchida, the plant-parasitic nematodes are found in two Super Families: Tylenchoidea and Criconematoidea. There are more than 100,000 described species of nematodes.
Currently, the most effective substance for soil treatment is methyl bromide. Methyl bromide is used in the control of pest insects, nematodes, weeds, pathogens, and rodents. In the United States, about 27,000 tons of methyl bromide is used annually in agriculture, primarily for soil fumigation, as well as for commodity and quarantine treatment, and structural fumigation. Globally, about 76,000 tons of methyl bromide are used each year.
When used as a soil treatment, methyl bromide is injected into the soil at a depth of 12 to 24 inches before a crop is planted. This will effectively sterilize the soil, killing the vast majority of soil organisms. Immediately after the methyl bromide is injected, the soil is covered with plastic tarps that hold most of the methyl bromide in the soil. The tarps are removed 24 to 72 hours later.
After the tarps are removed, much of the methyl bromide leaves the soil. The EPA estimates that about 50% to 95% of the methyl bromide in the soil eventually enters the atmosphere.
While methyl bromide in large doses can result in damage to the human nervous system and respiratory system, the greatest danger poised by methyl bromides is the damage to the ozone layer. According to the 1994 Assessment of Ozone Depletion, the Ozone Depletion Potential (ODP) of methyl bromide has been assessed to be 0.6. This makes the ODP of the methyl bromide fifty times more effective at destroying ozone than CFC's on a per molecule basis.
According to the Clean Air Act (1990 Amendments), all substances with an ODP of 0.2 or greater are to be phased out in the United States. This means that methyl bromide will need to be phased out. There has been legislation to ultimately prohibit the production and importation of methyl bromide in the United States. In addition, 160 countries have signed the Montreal Protocol, a treaty calling for the levels of ozone-depleting chemicals to be frozen at 1991 levels. Finally, the EPA is lobbying for nations to stop using methyl bromide all together.
In light of the environmental problems with methyl bromide, and the continuing need for a soil treatment, an environmentally safe chemical alternative has been sought. Thus, there remains a need for pathogen control methods which are more compatible with the need for affordable and effective disease control, a high degree of food safety, and minimal environmental impact.